The present invention relates to an optical system for EUV lithography, comprising a reflective optical element, including a substrate with a highly reflective coating emitting secondary electrons when irradiated with EUV radiation, and a source of electrically charged particles, which is arranged in such a manner that electrically charged particles can be applied to the reflective optical element. Moreover, the present invention relates to an illumination system for EUV lithography, a projection system for EUV lithography, and an EUV lithography apparatus having such an optical system.
In EUV lithography, for the lithography processing of semiconductor elements, reflective optical elements are used for the soft X-ray to extreme ultraviolet (EUV) wavelength ranges (e.g. wavelengths between about 5 nm and 20 nm) such as for photomasks or multilayer mirrors. Since EUV lithography apparatuses usually have several optical elements, they have to have the highest possible reflectivity to ensure sufficient overall reflectivity. Since a plurality of optical elements are usually arranged in series in an EUV lithography apparatus, even the slightest deterioration in reflectivity of any one optical element has severe repercussions for the overall reflectivity within the EUV lithography apparatus.
In the operation of EUV lithography apparatuses, reflective optical elements are exposed to as intense an EUV radiation as possible to keep exposure times as short as possible. In the interior of EUV lithography apparatuses, in particular in the interior of illumination and projection systems, vacuum conditions prevail. However, very small proportions of water, oxygen and hydrocarbons cannot be entirely eliminated in the residual gas atmosphere. These residual gases may be split into reactive fragments by the radiation, which can lead to contamination and deterioration of the surface of the highly reflective coating of the reflective optical elements. These reactive fragments can be generated either directly by the EUV radiation or by secondary electrons generated by EUV radiation. Two important processes, which corrode the surface of the reflective optical elements, are the oxidation due to the dissociation of adsorbed water molecules, and the growth of carbon layers due to the dissociation of adsorbed hydrocarbon molecules. Both processes can be caused by photoemitted secondary electrons. The actual maximum reflectivity of each of the reflective optical elements can be reduced by the contamination or oxidation of the top layer.
A common way to clean contaminated surfaces of reflective optical elements is to provide them with hydrogen radicals and ions. From U.S. Pat. No. 6,642,531 B1 it is known to remove particles from surfaces of reflective optical elements by providing a charge to them by showering them with electrons and moving them away from the reflective surface by electrostatic elements.
To reduce the influence of the secondary electrons, electrode arrangements are known in which the reflective optical element is connected as the first electrode, and a second electrode is arranged at a distance from the irradiated surface of the reflective optical element to remove any produced secondary electrons before they contribute to carbon contamination or oxidation of the reflective optical element.
Another approach to reduce the influence of the secondary electrons is to earth the reflective optical elements to prevent the surface of the reflective optical element from being uncontrollably electrically charged. Electrical charging has the drawback that the charged surface attracts electrically charged fragments having the opposite electrical charge, which can lead to sputtering effects on the surface of the reflective optical elements. To achieve earthing of reflective optical elements for EUV lithography, wherein a substrate is provided with a highly reflective coating, either the highly reflective coating or the substrate can be contacted to achieve charge carrier compensation. If, however, the substrate partially or wholly consists of an insulating or poorly conductive material, sufficient charge carrier compensation cannot occur to avoid electrically charging the optical surface.